Previous studies suggested that the estrogen antagonist Tamoxifen may reduce the incidence of second primary breast cancers in women receiving this medication as adjuvant therapy for early stage breast cancer. The Breast Cancer Prevention Trial (BCPT) is a multi-center trial designed to determine whether administration of Tamoxifen to women at increased risk of developing breast cancer will decrease the incidence of and mortality due to breast cancer in this population. The University of Michigan was selected as one of the centers participating in this trial.